1. Field of the Invention
The present invention relates generally to an apparatus and method for protection of fragile sensors. More particularly, it relates to a system using a buffer insert for improved protection of transducers used in the oil and gas industry.
2. Description of Related Art
Downhole memory gauge systems may be used to measure, record, store, and/or transmit information concerning environmental conditions and physical phenomena, such as temperature and pressure, in locations within and about a wellbore. In many cases, the information is important for establishing and regulating operating parameters for downhole procedures. Known gauge systems typically employ one or more sensors that are capable of sampling a particular condition, such as temperature or pressure, and means for recording and storing or transmitting this information for interpretation at the surface. More advanced gauge systems include features for monitoring changing well conditions, conserving power, and for evaluating the sensor's own status. Some gauge systems are self-contained in that they obtain and store information within themselves for use only after the system has been extracted from the wellbore. Others are capable of transmitting information to remote locations for real time readouts. Commonly, this will be surface readout of downhole well conditions.
A popular and effective pressure sensor used in the oil and gas industry is a quartz crystal transducer that relays signals via gold conductor strips to insulated copper transmission wires. Information about well conditions would be most accurately gathered by emersing the crystal directly in the wellbore fluids. Contact of the transducer with wellbore fluids may, however, invalidate the readings and damage the transducer. The crystal, gold strips, wires and epoxies used to connect the gold strips to the wires are susceptible to damage from chemicals and contaminants found in wellbore fluids, such as H.sub.2 S, Other sensor types include sensitive components that may be similarly harmed.
Oil and grease filled chambers have historically been used to safeguard crystal transducers. The transducer is emersed in the oil and grease chamber and located therein. The oil and grease will not harm the crystal and therefore provides an effective barrier to the harmful fluids. These more viscous and substantially incompressible fluids are retained within the chamber by the naturally occurring capillary attraction between the oil and grease and the walls of the chamber.
Protection against wellbore fluids is particularly important in systems that are self-contained and may remain downhole for extended periods of time. Over time, wellbore fluids tend to infiltrate gauge systems and reach the components of the transducer. Fluid may infiltrate the gauge systems by physically displacing protective oil surrounding the transducer or contaminants and gases may dissolve into the surrounding oil and migrate to the crystal.
In current systems, a crystal transducer acting as a sensor is placed within a chamber that is connected to a buffer system. The buffer system is covered with a surrounding outer housing having an interior that defines a buffer chamber. The crystal chamber and the buffer chamber are in fluid communication with the wellbore therefore the sensor may be exposed to the potentially harmful external conditions to be monitored. The silicon oil in the crystal chamber may be contaminated by wellbore fluids entering through the outer housing and passing through the buffer system. One buffer system includes a single, helical or curled capillary tube, known as a buffer tube, that is positioned adjacent to the crystal chamber and within the outer housing. The tube allows fluid communication between the wellbore and the interior of the crystal chamber. Capillary attraction between the oil and the interior walls of the tube slows progress of the wellbore fluid toward the crystal transducer. For contaminating fluids or solids to reach the crystal, they must either displace, dissolve into, or pass through the oil along the length of the capillary tube. This arrangement, however, is only effective to a limited degree in preventing wellbore contaminants from reaching the transducer components.
Alternatively, closed systems that eliminate the opening between the crystal chamber and wellbore are known. These systems incorporate an accordion-like folded metal bellows within the outer housing. Closed systems are less sensitive to wellbore parameters than open systems. They are also not field serviceable since it is not practical to service and fill the closed housing. Additionally, if the closed system is opened, re-calibration of the sensor contained therein may be necessary.